The main explanation and all the comments so far seem to have missed one of the main points of the design. Yes - cross fire was important, but consider someone attacking one of these things without the benefit of aerial surveillance or a map. One of their main advantages is that, from the point of view of a stranger approaching on the ground, it's just plain difficult to work out how to get in. Often you would have to work your way up between two stellations, only to discover that you'd picked the wrong pair and there wasn't an entrance there at all.

There's a really nice one in Lille, France - you have to pass through an outer ring of stellated defences (not knowing where you need to go, and probably with defenders dropping things on you from above) and then you're faced with another inner set of stellated defences.

This is why spies drawing maps of defences were so important in those days.